The conductor of a high voltage electric power cable generates heat when transmitting electric power. This heat is transferred through the cable insulation arranged around the conductor and the temperature in the surrounding area of the cable is increased due to those heat losses. The conductor is, for example, made of copper or aluminum, and the electric insulation referred to herein may be polymeric and then typically comprises cross-linked polyethylene, or an oil impregnated paper insulation. The heat generated in the conductor may lead to deterioration of the insulation if the temperature of the conductor is not maintained within a defined interval. One way of keeping the temperature of the conductor in the defined interval is to increase the conductor area. However, this is not desirable as the material used in the conductor is expensive and also an increased amount of electric insulation material will be required with regard to the increased conductor area.
For electric power cables laid underground there are different ways to handle the heat losses generated when transmitting electric power in the cable. It is, for example, possible to embed, in the soil adjacent to the cables, a pipe through which a cooling liquid could pass to maintain the temperature of the soil. Another way is to enclose the cable, or cables, in a pipe or duct through which a cooling medium, for example, air or water, is circulated. The cooling medium extracts the additional heat generated by the conductor and thereby keeps the temperature of the cable within the permitted temperature limits.
Patent specification GB 875,930 discloses a cable where a plurality of ducts or pipes are provided for the circulation of a cooling liquid in an outer impermeable protective covering or sheath of plastic material enclosing the sheath surrounding the one or several cable cores. Heat generated in the conductor when the cable is transmitting electric power is dissipated by the cooling liquid circulated through the pipes and the temperature of the cable is maintained within the permitted temperature limits.
Patent abstract JP 54-056187 discloses a power cable comprising a metallic or plastic cooling pipe arranged in a gap between cable cores of the cable. Cooling air or water is arranged in the cooling pipe to absorb the heat generated in the conductor of the cable core.
Patent specification EP 0562331 discloses an electric cable comprising three cable cores with integrated cooling by at least one jointly stranded cooling element with at least one conveying hollow duct for forward and backward flow where at least one coolant conveying cable element is constructed in the form of a composite section made of aluminum and having an inner pipe of steel for holding a cooling medium.